Following development of information technology, the information recording technique, in particular, the magnetic recording technique has remarkably advanced. In a magnetic disk to be used in a HDD (Hard Disk Drive) or the like, which is known as one of magnetic recording media, rapid improvement has continued towards a smaller size, a reduced thickness, an increased recording density, and a higher access speed. In the HDD, recording and reproducing operations are carried out by rotating, at a high speed, the magnetic disk comprising a disk-shaped substrate and a magnetic layer formed thereon and keeping a magnetic head flying above the magnetic disk.
Following an increase in access speed, the rotation speed of the magnetic disk is increased also. Therefore, the magnetic disk is required to have a higher strength of the substrate. Furthermore, following an increase in recording density, the magnetic head has shifted from a thin film head to a magnetoresistive head (MR head) and then to a giant magnetoresistive head (GMR head) and a flying height of the magnetic head from the magnetic disk has been reduced to about 5 nm. Under the circumstances, if a surface of the magnetic disk has an uneven shape, various defects such as a head crash failure and a thermal asperity failure may be caused to occur. The head crash failure is a failure in which the magnetic head collides with the magnetic disk. The thermal asperity failure is a failure in which a read error occurs due to heating caused by adiabatic compression of air or contact between the magnetic head and the magnetic disk. In order to suppress those failures caused at the magnetic head, it is important to finish a main surface of the magnetic disk into an extremely flat surface.
Under the circumstances, at present, a glass substrate has been increasingly used as a magnetic disk substrate instead of a conventional aluminum substrate. As compared with the aluminum substrate made of a metal as a soft material, the glass substrate made of a glass as a hard material is excellent in flatness of a substrate surface, substrate strength, and rigidity. The glass substrate for use in the magnetic disk is manufactured through grinding and polishing processes of grinding and polishing its main surfaces. The grinding and the polishing processes of the glass substrate may be performed by using a double-side polishing apparatus having a planetary gear mechanism.
As described in JP-A-2009-214219 (Patent Document 1), the planetary gear mechanism has upper and lower surface plates with polishing pads (polishing closes) attached thereto. A glass substrate is sandwiched between the upper and the lower surface plates. While a polishing liquid (slurry) containing abrasive grains suspended therein is supplied between the polishing pads and the glass substrate, the glass substrate is moved relative to the upper and the lower surface plates. Thus, each of the main surfaces of the glass substrate are finished into a flat surface.
In one aspect, the glass substrate is a brittle material. Therefore, in a manufacturing process of a magnetic disk glass substrate, the glass substrate is dipped into a heated chemical strengthening liquid so that lithium ions and sodium ions in a surface layer of the glass substrate are replaced or ion-exchanged by sodium ions and potassium ions in the chemical strengthening solution, respectively. Thus, a compressive stress layer is formed in the surface layer of the glass substrate to strengthen the glass substrate (glass strengthening step).
In the manufacturing process of the magnetic disk glass substrate, in addition to flattening by the grinding and the polishing processes, it is also an important problem to remove even a little contamination on the surface of the glass substrate so as to keep the surface of the glass substrate clean. It is known that, in order to keep the substrate surface finally clean, the glass substrate is cleaned under an acidic condition after each of the grinding and the polishing processes.
Then, on the magnetic disk glass substrate with its surface flattened by the grinding and the polishing processes, a thin film (magnetic layer) having a thickness on the order of several nanometers is formed and then a recording/reproducing track is formed.